This invention relates to an actuator comprising an electric induction motor, and also to an electric motor drive system comprising an electric motor and a transmission. The invention is particularly useful in electrically driven actuators used to operate valves or penstocks in, typically, fluid transfer systems, the actuators being of the type and range as described in British Patent No. 2196494B and is concerned, primarily, with actuators in which the drive motor is of the polyphase or single phase induction type.
The aforementioned British Patent No. 2196494B describes a method of measuring the torque in the worm gear drive to the output column by giving a limited degree of axial freedom of movement to the wormshaft, which is also integral with the motor shaft, and displacing this shaft from a neutral position will compress one or other sets of springs. The springs, having a relationship between compression and spring force, will therefore cause the shaft displacement to be a function, for example a linear function, of the output torque. This movement can then be converted into an analogue electrical signal by means of a potentiometer or "Hall Effect" device or other proximity sensor.
This present mechanical method of measuring the torque produced at the column suffers from a number of limitations which have become more apparent as the various other monitoring and control functions of the actuator have been developed using direct electronic analogue and digital signal techniques. Firstly, the spring packs require to be set to a pre-load and calibrated--a task which can only be performed satisfactorily at the time of initial manufacture.
Secondly, the motor has to be constructed using a special frame to accommodate the axial movement of the shaft and cannot be removed from the actuator without disturbing the initial spring settings.
Thirdly, it is found, in practice, that the available operating torque range of any particular frame size of actuator is larger than the range achievable by a single design of spring pack--resulting in the need to provide two or more sets of spring packs to cover the whole range of output torques from a single design of actuator.
It has been proposed that the aforementioned limitations could be overcome by replacing the existing mechanically derived torque signal, in an actuator driven by a polyphase or single phase induction motor, by an accurate torque signal derived by electronic means.